Friction couplings of different types are today well known and widely used. One example of a friction coupling is disclosed in U.S. Pat. No. 4,362,411 which shows a one-sided friction coupling arranged to press a first element towards a second element by means of a pressure medium. The pressure medium is arranged to be pressed into several pockets, which are provided separate from each other in a wall of a bore of the first element, by means of a piston. When the pressure medium is pressed into the pocket, an element provided in the pocket is displaced radially, pressing the formable inner wall of the bore in towards the second element, thereby creating a friction coupling. A drawback with this known friction coupling is that it is difficult to provide an even distribution of the pressure medium and thereby to provide an even pressure along the entire first and second element which may result in a warped friction coupling causing undesired wear.
Other known friction couplings use techniques like gluing and clenching when elements of the friction coupling are about to be joined together. A major disadvantage with these techniques is that they do not achieve the desired rigidity or strength-weight ratio. Also, if using clenches the element has to have a recess to receive the clench which may change and deteriorate the properties of the material.
In many other designs the joining between composite materials is the weakest link in the construction and one possible reason for this is that the materials used today, e.g. steel, are heavy causing the joining to break.